You need something :
-that does not bend, hard enough so the surface does not move. Depending on your rig, you don’t want the surface to move at all while you are scanning. And, if you are using a pre-aligned workflow, you don’t want the mat to move between shoots, which is even harder.
-with enough features : you can use any unique pattern to help RC to align. You also need to include known distances so you can scale your object. Your features need to be easily pointable on pictures : a line is not that good, a cross is better…
(I had very good results with chromatic noise generated in GIMP (over black for projection or over white for printing) + a few named contrasting cross to have easily pointable control points)
-you could use the new RC generated coded markers so you can automate the control points part. I didn’t try this but this is planned.
If you really want to make the editing easier, you could ask your people to stand on a piece of wood to add 1 cm on top of the mat, so this is really easy to filter out the 3D object/people without cutting the bottom of the shoes / feet.
One last thing, make sure you are perfectly level (horizontal) with a bubble level. You can use some height-adjustable feet to make sure your mat is perfectly leveled. Because your points on the map will be real ground control points, so they need to be on the ground plane.
Of course, not absolute rules, this is all about experiment and try and error !